1. Field of the Invention
The present invention relates to a visible ray communication system and a method for transmitting a signal therein.
2. Description of the Related Art
Optical communication technology using a visible light region is receiving more attention. For example, researchers are investigating an illuminating device, such as a Light Emitting Diode (LED), as a potential technology for achieving high speed data communication.
As a light emitting device for high speed optical data communication, an LED is a leading candidate in consideration of an influence on a human body or medical appliances. However, a semiconductor light emitting element with a higher speed response performance, such as a Laser Diode (LD) or a Super Luminescent Diode (SLD), is another proposed candidate. The data transmission speed in optical communication depends on a response speed of a light emitting element. Accordingly, light emitting elements with higher speed response performances are currently receiving more attention.
Also, in order to further improve the data transmission speed, a technology for stably transmitting large amounts of data in one signal emitted from a light emitting element is desired. For example, Japanese Patent Laid-Open Publication HEI 2008-252444 discloses a technology for preventing spatial interference by allocating a time axis of an Orthogonal Frequency-Division Multiplexing (OFDM) signal in a spatial direction.
If an OFDM scheme is used, it is possible to improve the frequency use efficiency and multi-pass resistance. Therefore, the OFDM scheme is widely used in a wireless communication system (e.g., a wireless Local Area Network (LAN)) or a wired communication system (e.g., Asymmetric Digital Subscriber Line (ADSL)). An OFDM scheme is also expected to improve the communication quality in the visible ray communication. However, there is a problem of increasing a Peak to Average Power Ratio (PAPR) in the OFDM scheme. That is, a large dynamic range is required in a transmitter and receiver.
Accordingly, if the OFDM scheme is applied to the visible ray communication utilizing an LED, a large amount of current is supplied to the LED. For example, current of several 100 mA to several A flows to the LED. Therefore, a drive circuit is then installed, which is capable of handling a signal having a wide dynamic range on a transmission side. However, a conventional LED is designed for emitting light of a certain light quantity. Therefore, requiring a specific device to handle the signal of the large dynamic range is inconvenient, if even possible.
If the OFDM scheme is applied to the visible ray communication, as much transmission data as the number of carriers are generated through serial-to-parallel conversion in the transmission side. Each of the transmission data is allocated to a carrier signal to be added, and light emission of an LED is controlled in a light-emitting strength according to signal amplitude, after the corresponding addition.
Further, if a plurality of LEDs of different light-emitting colors (light frequency) is used, there are concerns on influence of inter-color interference generated by a frequency property of the LEDs and Photo Diodes (PDs), but there is an orthogonal property between the carriers as described above so that the influence by the inter-color interference is prevented. Therefore, it is possible to increase the number of multiplexing operations (the number of colors) by a certain degree and improve the transmission rate. However, the frequency between the colors is adjacent according to the increase of the number of multiplexing operations and thus the influence of inter-color interference increases, such that there is a limitation in an effect of transmission rate increase through the increase of the number of multiplexing operations. For this reason, in order to more improve the transmission rate, technology for transmitting more data with the same number of multiplexing operations is required.